Motor vehicles

ABSTRACT

A machine for drilling apertures in sheet metal work pieces of the type adapted for fabricating buses. The sheet metal is clamped against an inverted-U framework by straps. A movable, inverted-U, outer frame carries a plurality of inwardly extending drills for drilling holes in the sheet metal. The outer frame intermittently moves parallel to the stationary framework, by automatic indexing means, so that the drills may drill at selected, predetermined points along the length of the sheet metal work pieces. An inner frame member carries driving means for translating itself and, through drive pins, also drives the outer frame member along its path. The drive pins from the inner frame member leap-froggingly engage the outer frame member with respect to spaced uprights which support the sheet metal carrying framework.

United States Patent [191 McGowan [4 1 Feb. 19, 1974 1 MOTOR VEHICLES [75] Inventor: Joseph McGowan, Preston, England [73] Assignee: British Leyland Truck and Bus Division Limited, Leyland, Lancashire, England.

[22] Filed: Aug. 20, 1971 [21] Appl. No.: 173,614

[30] Foreign Application Priority Data Primary ExaminerGranvi1le Y. Custer, Jr. Attorney, Agent, or FirmThomas J. Greer, Jr.

[57] ABSTRACT A machine for drilling apertures in sheet metal work pieces of the type adapted for fabricating buses. The sheet metal is clamped against an inverted-U framework by straps. A movable, inverted-U, outer frame carries a plurality of inwardly extending drills for drilling holes in the sheet metal. The outer frame intermittently moves parallel to the stationary framework, by automatic indexing means, so that the drills may drill at selected, predetermined points along the length of the sheet metal work pieces. An inner frame member carries driving means for translating itself and, through drive pins, also drives the outer frame member along its path. The drive pins from the inner frame member leap-froggingly engage the outer frame member with respect to spaced uprights which support the sheet metal carrying framework.

3 Claims, 9 Drawing Figures FATENTED FEB 1 9 I974 SHEET 8 BF 8 MOTOR VEHICLES The present invention relates to motor vehicles and more particularly to the manufacture of an integral construction public service vehicle.

It is known to manufacture integral construction single-deck public service vehicles using common subassemblies to enable more than one length of vehicle to be produced (Automotive Design Engineering, Dec. I965, Page 72). In this known method the subassemblies are jig built by welding to form the underframe, side, front end, rear end and the roof assemblies, which are then brought together in a fixture and welded together to form a skeleton body shell. The various smaller items such as wheel arches are then secured in place. The shell is then treated against rust and the outer skins or panels are pop-riveted or spot welded in place. The body is thenpainted with an anti-rust paint, baked, and finally painted with a top coat. After the fitting-out of the body with the usual equipment, it is lowered onto its running gear, i.e., front and rear axle/ suspension assemblies which have already been positioned on the moving track of the production line. The engine/gear box unit is then fitted.

The disadvantage of the above known method of construction is that because the units and subassemblies are welded together subsequent repairs to the body are expensive and time consuming to carry out.

According to the present invention in a system for constructing a public service vehicle having an integral body, units and sub-assemblies making up the body shell are riveted together.

According to a first aspect of the invention each rivet which secures a load bearing unit or sub-assembly is accessible and where any unit or sub-assembly covers the said first mentioned unit or sub-assembly an aperture is formed in the second mentioned unit or sub-assembly to enable access to the rivet through the second unit or sub-assembly. This arrangement enables the rivets to be stripped, to enable repairs to be effected, without any structural panels having to be removed.

According to a second aspect of the invention sets of rivet holes are formed and associated rivets are secured at a fixed pitch along the length of the body, i.e., each set of rivets is spaced from its adjacent set of rivets by the same distance measured longitudinally of the body.

According to a third-aspect of the invention integral public service vehicle units or sub-assemblies are riveted together by an apparatus which has means to index it lengthwise of the vehicle so that sets of rivets equidistantly spaced from one another lengthwise of the vehicle can be riveted into the units or subassemblies.

According to a fourth aspect of the invention the apparatus includes a movable carriage which is indexable along the unit, sub-assemblies or complete body shell, the carriage carrying drills or rivet guns by means of which the units, sub assemblies or complete body shell can be drilled or riveted.

According to a'fifth aspect of the invention the apparatus includes a bridge which is adapted to span the unit, subassembly or complete body shell to be drilled or riveted, the bridge being indexable longitudinally of the unit, sub-assembly or complete body shell, and carrying drills or rivet guns by means of which the unit,

sub-assembly or complete body shell can be drilled or riveted.

According to a sixth aspect of the invention the apparatus includes an upper carriage adapted to pass over the top of a unit or sub-assembly to be drilled or riveted, and a lower carriage adapted to pass beneath the unit or sub-assembly to be drilled or riveted, each of the carriages carrying a plurality of drills or rivet guns.

According to a seventh aspect of the invention, the lower carriage is provided with a motor by which it can be driven and is connectable to the upper carriage by retractable pins in order to drive the upper carriage.

According to an eighth aspect of the invention each of the said drills is a self-feed drill.

According to a ninth aspect of the invention the two carriages are supported by two rows of upright support members, each support member serving to support first and second tracks along which the upper and lower carriages are adapted to run respectively, the first tracks being positioned outside the second tracks, the rigid frame for supporting the workpiece being carried by posts which are located between the first and second tracks the lower carriage having two pairs of retractable drive pins so that as the lower carriage approaches one of the said posts one pair of pins can be retracted to clear the posts while the drive is maintained through the other pair of drive pins.

According to a 10th aspect of the invention the supports also have means associated with them for tensioning a plurality of steel strips over the unit or subassembly which is to be drilled and riveted in order to hold the unit or sub-assembly firmly in position during the drilling and the riveting operations.

According to the llth aspect of the invention the supports also carry program bars which serve to control the indexing of the two carriages along the length of the unit or sub-assembly which is to be drilled or riveted.

According to the 12th aspect of the invention the upper carriage carries index pins which are adapted to be inserted into apertures in a program bar extending alongside a first track in order to locate the upper carriage on the various drilling stations along the workpiece.

According to a 13th aspect of the invention the drilling carriages referred to above are duplicated by similar carriages which carry rivet guns by means of which rivets can be inserted in the holes previously drilled by the drills carried by the drilling carriages.

According to a 14th aspect of the invention each of the drilling units has a spring loaded nose through which the drill itself is adapted to pass, the drill being contained within the nose when not in operation, and the nose being adapted to be brought into contact with the workpiece and the drill to be extended out of the nose in order to carry out the drilling operation.

How the invention may be carried out will now be described by way of example only with reference to the accompanying drawings in which:

FIG. 1 is an exploded diagrammatic representation of the various sub-assemblies and units which make up the body shell of a public service vehicle constructed according to the present invention;

FIG. 2 is a fragmentary view, on an enlarged scale, showing a portion of the body shell;

FIG. 3 is an end view of drilling jig used in the construction of the bus shown in FIG. 1;

FIG. 4 is a plan view of the drive arrangement of the lower carriage;

FIG. is a diagrammatic representation of the controls for the motor used to drive the lower carriage;

FIG. 6 is a diagrammatic representation of the drive connection between the upper and lower carriage;

FIG. 7 is an enlarged plan view of the index pin arrangement shown in FIG. 3;

FIG. 8 is an elevational view of FIG. 7; and

FIG. 9 is a part sectional view of a spring loaded nose for a drive unit.

FIG. 1 l

A single deck public service vehicle is built up essentially from an underframe 1, two body sides 2 and 3 respectively, a front end 4, a rear end 5 and a roof 6. The underframe 1 is constructed from pressed steel units, some of which are welded and some of which are riveted together.

The sides 2 and 3 are formed from pressed steel units which are assembled in a jig and riveted to one another to form a sub-assembly. The front and rear ends 4 and 5 and the roof 6 are similarly constructed.

In the manufacture of the vehicle the underframe is constructed and then transferred to a moving track. At the same time the two body sides 2 and 3 are constructed in jigs as sub-assemblies, as are the front and rear ends 4 and 5.

The ends 4 and 5 are then key riveted to the underframe to locate them in position and the two sides 2 and 3 are then also brought into position on the underframe 1.

The underframe then passes along the track and the front and rear ends 4 and 5 and the two sides 2 and 3 are riveted to the underframe and to one another.

While the latter steps are being carried out the roof assembly is being constructed and the roof is then assembled on the open box" structure formed by the underframe, the front and rear ends and the sides of the vehicle.

The body shell then passes along the track to have the various smaller units secured in position, such as wheel arches, doorway steps and driver's compartment.

The completed body shell is then'undersealed and painted, the various units making up the body having been pre-treated against corrosion before being riveted together to form the various sub-assemblies.

FIG. 2

The structural units 8 making up the vehicle body are provided with rows of apertures 7 which enable the rivet holes to be accessible for securing rivets in posi tion during assembly of the vehicle and which also enable the rivets to be stripped by a stripping tool if it becomes necessary to repair the vehicle. The circled portion denoted by A of FIG. 1 is depicted in detail at FIG. 2. With this construction it is unnecessary to remove the inner structural members 8 in order to gain access to the rivets R which secure the outer structural panel 9 in position. A considerable time saving is thus effected in both building the vehicle and also in any consequent repairs to it.

FIGS. 3 TO 9 In constructing the sub-assemblies, such as the body sides 2 and 3 and the roof assembly 6, the apparatus used comprises essentially a support on which the units to be drilled and riveted are mounted and held firmly in position, and a moving carrier on which drills and/or rivet guns are mounted, the carrier being designed to traverse the support in order to bring the drills and/or rivet guns into successive positions along the units to be drilled and riveted. The carrier has means for indexing it lengthwise of the sub assembly so that rows of rivet holes are formed along the subassembly.

A specific example of such apparatus is shown generally in FIG. 3, this apparatus being designed to drill the roof 6 of a bus of either ten or eleven metres length.

The apparatus comprises two mutually parallel rows of supports and 101 which carry a rigid framework 109, on uprights 110, adapted to support the assembly to be drilled. The supports carry an upper carriage 102 and a lower carriage 103. The carriage 102 runs on four castor wheels 104 along an outer track 105 carried by the supports 100 and 101. The carriage 103 runs on four castor wheels 106 along inner tracks 107 mounted on the supports 100 and 101.

Each of the carriages 102 and 103 carries a number of self-feeding air-operated drills 108 which are positioned across the carriages so that they will form holes at predetermined positions across the panels which make up the roof assembly, generally indicated by the reference numeral 6.

The panels making up the roof assembly 6 are held firmly in position on the rigid framework 109 by means of a plurality of metal straps 112 which have their ends connected to tensioning arrangements 113 carried by the uprights 110. One end of each of the straps 112 is secured to a fixed point denoted by B of FIG. 3., while the other end is connected to any conventional tensioning device, such as, for example, a toggle device 113.

The control system for the carriages 102, 103 is such that the two carriages move in unison and start from one end of the roof assembly and move longitudinally to the other end of the roof assembly in a series of steps, at the end of each step the drills carried by the two carriages being brought into operation to drill the roof assembly. After drilling has taken place rivets may be inserted into the drilled holes manually or the upper and lower carriages may be duplicated by a second set of carriages having rivet guns carried on them. With this arrangement the second set of carriages would follow the first-mentioned set of carriages in order to secure the rivets in the holes previously drilled by the drilling carriages.

The arrangement for driving the carriages 102 and 103 will now be described with reference to FIGS. 4 to 8.

FIG. 4

The lower carriage 103 is provided with an electric motor 121 which is adapted to be drivably connected to a driven axle 128 through a transmission arrangement.

The transmission arrangement comprises a flexible coupling 122, a variable drive unit 123, a magnetic clutch 124, worm reduction gearing 126 and flexible couplings 127. A magnetic brake 129 is provided in order to retard the lower carriage.

A handwheel 130 is drivably connected to a shaft 125, between the clutch 124 and the reduction gearing 126, by a shaft 131 and gears 132, 133 to enable the axle 128 to be manually driven if necessary.

I FIG. 5

The variable drive unit 123 is controlled by a lever 134 which when moved to the left, as viewed in FIG. 5, causes the motor 121 to drive the lower carriage 103 in a forward direction and when moved to the right, as viewed in FIG. 5, causes the motor 121 to drive the carriage 103 in the reverse direction. The extent to which the lever 134 is moved to the left or right, away from the neutral or starting position illustrated, determines the speed at which the carriage 103 is driven.

The variable drive unit comprises essentially a piston which ismovable in a cylinder which has a fluid outlet connected to a hydraulic motor. The reciprocation of v the piston in the cylinder is controlled by a swash-plate control the drive. Such variable drive units are well known and are shown, for example, in US. Pat. No.

The movement of the control lever 134 is effected directly by two servo pistons 135 and 136 which in turn are controlled by solenoid operated valves 137 and 138.

The piston 135 divides a cylinder 139 into first and second chambers 140 and 150 respectively and the pis' ton 136 divides a cylinder 151 into third and fourth chambers 152 and 153 respectively. The piston 135 is connected by a rod 154 to the lever 134, at an intermediate point along its length, and the piston 136 is connected by a rod 155 to the carriage 103. The cylinders 139 and 151 are back-to-back.

Air can be supplied to the chambers 140, 150 and 152, 153 through conduits 156, 157, 158 and 159 respectively viathe valves 137 and 138 which are controlled by solenoids 160, 161 and 162, 163 respectively.

In the position illustrated in FIG. 5 the solenoids 160 and 162 are energized to hold the lever 134 of the drive unit 123 in neutral. To move the lever 134 into the forward position solenoids 161 and 162 are energized, and to move the lever 134 into the reverse positionsolenoids 160 and 163 are energized. In the solid line position of the lever 103 shown in FIG. 5, i.e., in neutral, pressurized fluid passes from the reservoir 171 through conduits 156 and 158 into chambers 140 and 152. Because the piston 136 is fixed by the rod 155 the result of the pressurized fluid in 152 is to move the cylinder 139 bodily to the left. However, the simultaneous application of pressure in the chamber 140 tends to move the piston 135 to the right. The resultant of these two movements (which are indeed opposite in direction) is that the rod 154 does not move at all thus leaving the control lever 103 stationary in the neutral position illustrated. When the solenoids 161 and 162 are energized (i.e., to produce forward motion of the carriage 103) the valve 137 is moved to the left thus bringing the conduits 157 into communication with the fluid supply 171; the valve 138 is held in the same position as if neutral had been selected since the solenoid 132 remains energized. The result of this is to pressurize the chamber 150 thus tending to move the piston 135 to the left; the pressure already being applied to the chamber 152 through conduit 158 retains the cylinder 139 in the position illustrated in FIG, 5. Thus, the net result is that the rod 154, and hence the control lever 103, is

moved to the left into the dotted line position shown. If the solenoids and 163 are now energized, in order to produce reverse direction of the carriage 103, the conduits 156 and 159 are brought into fluid communication with the fluid reservoir 171. This results in pressurization of the chamber 140 and a tendency for the piston 135 to move to the right. It also results in pressurization of the chamber 153 resulting in a tendency for the cylinder 139 to also move to the right. The net result of these movements is for the rod 154 and hence the control lever 134 to move to the right into the dotted line position shown in FIG. 5. It should be remembered that the cylinders 139 and 151 are back-to-back, Le, a fixed wall separates them and is denoted by C in FIG. 5.

The movement of the control lever 134 is damped by a dashpot arrangement which comprises a piston 164 slidable in a cylinder 165 and connected to the free end of the lever 134 by an arm 166. The piston 164 divides the cylinder 165 into fifth and sixth chambers 167 and 168 respectively which are interconnected through two restrictors 169 and 170. The restrictor 169 allows throttled air flow from left to right, and free flow from right to left, whereas the restrictor 170 allows throttled air flow from right to left and free flow from left to right. The air is obtained from a reservoir 171.

FIG. 6

The lower carriage 103 drives the upper carriage 102. Because however the uprights 110 of the rigid frame 109 are located between the two parallel tracks 105 and 107 on which upper and lower carriages 102 and 103 run, it is necessary to provide a drive arrangement between the lower and upper carriages which is not interrupted as the lower carriage 103 passes the uprights 110.

The lower carriage 103 has two pairs of retractable drive pins 172, 173 and 174, 175 respectively which are adapted to engage in sockets 176 to 179 respectively in the upper carriage 102 in order to drive the latter.

These two pairs of driving pins can thus leapfrog the stationary uprights 110. As an upright is approached the leading pair of drive pins is retracted, leaving the drive from the lower to the upper carriage through the trailing pair of drive pins. When the leading pair of drive pins has passed the pair of uprights they are extended again into their associated sockets in the upper carriage and the trailing pair of drive pins are retracted to clear the pair of uprights.

In FIG. 6 the pair of drive pins 172, 173 are shown extended into the sockets 176, 177 to thus drive the upper carriage 102, and the other pair of drive pins 174, 175 are withdrawn to clear a pair of fixed uprights 110.

The drive pins 172, 173 are connected to two pistons 180, 181 by connecting the rods 182, 183 respectively. The pistons 180, 181 are slidable in the two cylinders 184, 185 which are back-to-back with respect to one another.

Air can be supplied to the cylinders 184, 185 either through conduits 186, 187 or through conduits 188, 189 to either retract or extend the drive pins 172, 173 respectively.

The conduits 186, 187 are fed from a common conduit 190 and the conduits 188, 189 from another common conduit 191. The conduits 190, 191 are in turn supplied with air from the reservoir 171 through a valve 192.

Movement of the spool of the valve 192 is controlled by two pilot air supplies 193, 194 in order to move it into one of the following three positions indicated in FIG. 6, namely:

where the supply of air from the reservoir 171 is shutoff;

where the supply of air from the reservoir 171 is connected to the conduit 191 and the conduit 190 is exhausted; and

where the supply of air from the reservoir 171 is fed to the conduit 190 and the conduit 191 is exhausted. I

There is a similar arrangement for extending and retracting the second pair of drive pins 174, 175 and for convenience the corresponding parts have been marked with the same reference numerals but with the suffix a added.

As the carriages move along the tracks 105, 107 the retraction and extension of the drive pins 172, 175 is effected automatically by means of a program bar 141, see FIG. 3, which carries two-sets of cams 117 and 118 associated with the travel of the carriages in both directions down the tracks 105, 107. Micro switches 119, 120 are associated with the cam sets 117 and 118 respectively and are energized by the cams to cause air to be supplied to the lines 193, 194, 193a, 194a in an appropriate sequence in order to retract and extend the drive pins 172 to 175. The micro switches 119, 120 control the passage of air in lines 193, 194, 193a, 194a by any desired manner, as for example, by controlling values in these lines.

FIGS. 7 AND 8 If one considers the progress of the two carriages 102 and 103 between an adjacent pair of drilling stations the following sequence of operations occurs:

1. drive of the lower carriage 103 is initiated manually by pressing a press button, or by pressing several interlocked press buttons 142, to cause energization of the solenoids 161 and 162, depression of the press button or press buttons also causing index pins 115 mounted on the carriage 102 to be lowered by air cylinders 197 in order to cause rollers 215 mounted on the index pins 115 to bear against program bars 116 having two sets of bushed apertures 195 and 196;

2. the carriages accelerate to a steady speed;

3. a probe or abutment P carried by the framework, such as by the stationary uprights 110 (see FIGS. 3 and 4), trips the solenoids 160 and 162 to disconnect the drive to the axle 128 and thus cause the lower carriage 103 to coast;

4. the carriages coast substantially to rest at the next drilling station but as they approach it the index pins 115 drop into the appropriate apertures 195 or 196 in order to locate the carriages in position for the next drilling operation.

The index pins 115 and program bars 116 will now be described in more detail.

In order to accurately locate the upper and lower carriages 102 and 103 with respect to the assembly to be drilled, program bars 116 having bushed apertures 195 and 196 are located along the length of the structure, outside the outer track 105. Index pins 115 carried by the upper carriage 102 are adapted to be inserted into the apertures 195 or 196 (depending on whether 10 or 1 1 metre buses are being built) in order to firmly locate the upper carriage 102, and hence the lower carriage 103, through the pins 172 to 179, in the correct position with respect to the roof assembly to be drilled.

The sets of holes 195 and 196 are to control the drilling of the roofs of a 10 or 1 1 metre length bus respectively. The index pin is movable axially by an air cylinder 197 to urge the pin 115 into an aperture in the bar 116. The index pin 115 and cylinder 197 assembly is mounted on a carrier 198 to enable the pin 1 15 to be brought into alignment with either row of apertures 195 or 196 depending upon whether a ten or eleven metre roof assembly is to be drilled. This is affected by means of an air cylinder 199 operating through a lever 201.

The cylinder 199 is fed with air through either conduit 205 or 206 in order to extend or retract the rod 200. The conduits 205, 206 are connected to valve 202 which is controlled by solenoids 203 and 204, which in turn may be actuated by, for example, a switch. Energization of solenoid 203 moves the valve into the position illustrated in which the conduit 205 is connected to the air reservoir 171 and the conduit 206 is connected to the exhaust. Energization of solenoid 204 moves the valve spool to the left to connect the conduit 206 to the air reservoir 171 and the conduit 205 to exhuast.

The air cylinder 197 can be supplied with air from the reservoir 171 through either of two conduits 210 or 211, via a valve 207 which is controlled by pilot air supplies 208 and 209. Air supplied through the conduit 108 causes the conduit 210 to be connected to the reservoir 171 and the conduit 211 to be exhausted thus urging the index pin 198 downwardly. Air supplied to the conduit 209 causes the conduit 211 to be connected to the reservoir 171 and the conduit 210 to be exhausted, thus urging the index pin 115 upwardly. In the position illustrated an extension 212 of the piston (not shown) in the air cylinder 197 makes contact with electrical contact 214, and when the index pin 115 drops into an aperture 195 or 196 the extension 212 makes contact with a second electrical contact 213, which is part of the control circuit for initiating the drilling operation.

FIG. 9

Each of the drill units 108 includes a chuck 216 which holds a drill 217. The chuck 216 is surrounded by a nose unit 218 which comprises a sleeve 219 and a smaller diameter extension 220.

The sleeve 219 is slidable on the main body of the drill unit and is urged to the right (as viewed in FIG. 9) by a compression spring 222 into the position illustrated. The extension 220 carries a bushing 221 through which the drill 217 can slide.

When the drill unit is inoperative the drill 217 is in the position illustrated. After the index pins 115 have dropped into the appropriate apertures 195 or 196 to locate the carriages 102 and 103 with respect to the workpiece, the noses 218 of the drill units are moved axially towards the workpiece to bring the extensions 220 into contact with the workpiece to clamp the latter and firmly locate the drill unit with respect to it. Further axial movement of the drill noses 218 compresses the spring 222 to cause the sleeve 219 to slide with respect to the extension 220 and cause the drill 217 to slide through the bushing 221 and drill a hole in the workpiece.

The advantage of the unit shown in FIG. 9 is that it both clamps the workpiece in position and also effects the drilling operation, instead of having to employ separate drilling and clamping units.

I claim:

1. An apparatus for drilling apertures such as rivet holes through sheet metal work pieces of the type adapted for buses, including,

a. an outer, parallel pair of tracks,

b. an upper, wheeled carriage movable on said outer tracks, and adapted to carry drills,

c. an inner, parallel pair of tracks, located between and parallel to said outer tracks,

d. a lower, wheeled carriage movable on said inner tracks, and motor means carried thereby for moving said lower carriage along said tracks,

e. a rigid frame for supporting a sheet metal work piece, said frame supported on longitudinally spaced, upstanding supports, said supports being positioned between said inner and outer tracks,

f. first and second pairs of driving pins, means for moving each pair, alternately, inwardly and outwardly, transverse to the said tracks, each pair lying on its own transverse axis, said pairs being longitudinally spaced from each other along said tracks,

g. said first and second pairs of driving pins carried by said lower carriage,

h. socket means carried by said upper carriage for receiving said driving pins when in their outward position,

i. whereby the inner carriage is connected to and drives the outer carriage along its track by means of alternate ones of the two pairs of driving pins, the driving pins being alternately retracted and engaged in the sockets to permit them to span the upstanding supports which support the rigid, work piece supporting frame.

2. The apparatus of claim 1 including,

a. strap means carried by said rigid frame for holding a work piece on said frame. I

3. The apparatus of claim 1 wherein, said motor means includes a variable speed transmission. 

1. An apparatus for drilling apertures such as rivet holEs through sheet metal work pieces of the type adapted for buses, including, a. an outer, parallel pair of tracks, b. an upper, wheeled carriage movable on said outer tracks, and adapted to carry drills, c. an inner, parallel pair of tracks, located between and parallel to said outer tracks, d. a lower, wheeled carriage movable on said inner tracks, and motor means carried thereby for moving said lower carriage along said tracks, e. a rigid frame for supporting a sheet metal work piece, said frame supported on longitudinally spaced, upstanding supports, said supports being positioned between said inner and outer tracks, f. first and second pairs of driving pins, means for moving each pair, alternately, inwardly and outwardly, transverse to the said tracks, each pair lying on its own transverse axis, said pairs being longitudinally spaced from each other along said tracks, g. said first and second pairs of driving pins carried by said lower carriage, h. socket means carried by said upper carriage for receiving said driving pins when in their outward position, i. whereby the inner carriage is connected to and drives the outer carriage along its track by means of alternate ones of the two pairs of driving pins, the driving pins being alternately retracted and engaged in the sockets to permit them to span the upstanding supports which support the rigid, work piece supporting frame.
 2. The apparatus of claim 1 including, a. strap means carried by said rigid frame for holding a work piece on said frame.
 3. The apparatus of claim 1 wherein, said motor means includes a variable speed transmission. 